In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Metallic resistance alloy is a dominant material used in the construction in electrical heating element assemblies. Typical FeCrAl alloys achieve their high temperature stability and long life by creating a protective oxide coating on the outer surfaces. This oxide layer contributes to the material's hot strength as well as protecting the core alloy from the formation of other oxides and nitrides that would rapidly consume the wire. The protective oxide layer is formed via the oxidation of aluminum inclusive in the heating alloy. One of the known properties of the FeCrAl resistance alloy is permanent elongation over time. Elongation is primarily caused when during thermal cycling of the alloy. The wire expands as it is heated, the oxide coefficient of expansion is less than the metal core, tensile stresses are created in the oxide coating and therefore cracks form in the oxide surface. The newly exposed alloy creates more oxide on the exposed areas and “heals” the surface. When the wire is cooled, compressive forces are created from the difference in thermal expansion from the alloy and the oxide. The compressive forces cause some of the oxide to flake or “spall” off of the material. Some portion of the elongation becomes permanent and the effect is cumulative over time.
Various improvements (such as powdered metallurgy) have been developed to minimize the permanent elongation characteristics of the alloy. It has been found that minimizing the stresses induced in the alloy helps reduce the elongation and generally extends element life. One source of stresses introduced into the wire is the force created when the helical coil of wire expands and pushes against the thermal installation surrounding the element assembly. Various approaches have been taken to attempt to mitigate this situation. Leaving a small space between the wire and insulation provides room for the coils to expand, but these designs do not address the issue of collinearity and concentricity of the coils. These prior art methods generally rely on some form of slot in the ceramic spacer rows that allow for expansion and contraction (as well as permanent elongation), but no mechanism is provided to insure the collinearity and concentricity of the coils. Since these assemblies are vertically mounted, gravity creates a downward force on the coil turns and encouraging the lower portions of the coil to increase in diameter, while the upper turns constrict. This can lead to increased forces applied to the bottom turns prior to the upper portion, leading to accelerated aging in the lower portions. Also, increased forces can be experienced at locations like the power terminals where the coils are somewhat fixed in location and the additional downward force from gravity is exerted. Some prior art attempts to remedy this situation by attaching protrusions to the heating element coils to block them from passing through the spacer assemblies. This can help and mitigate the accumulation of material in the lower part of the assembly but has negative implications to the heating wire temperature uniformity and potential risk of failure. Furthermore, these methods do not address the issue of keeping the coils collinear and centered. There is no constraining mechanism that keeps the coils collinear, therefore one coil can move horizontally relative an adjacent coil leading to irregular distribution of the heating element surface along the vertical axis. This can lead to decreased temperature uniformity within the heating element. Once deformation of the coil is initiated at some point in the assembly, it generally continues to worsen over time at that location. Therefore, the deformation can result in decreased element life as well.
Temperature uniformity and overall life can be affected by the centering of the coil within the assembly as well. The prior art does not provide a mechanism for maintaining the centering of the coil as well.
There is a need in the industry for an element assembly that allows the coil to move freely as it expands and contracts during thermal cycling while maintaining concentricity, collinearity and centering of the heating element coil.